The invention relates to a pump plug for use in well operations such as for displacing well tools via a flowline into a well, or for well swabbing purposes, examples of which are shown in U.S. Pat. Nos. 3,031,241 and 3,352,212, and German Pat. No. 1,811,038. The first type of operations is also referred to as through-the-flowline operations (which operations are indicated hereinafter by the expression "TFL-operations").
In TFL-operations, tools are transported from a site on land or a location on an offshore production platform via a flowline extending along the sea bottom or ocean floor to a submarine well. The flowline communicates with the production string in the well in a manner such that the tools can be transported via the flowline into the production string and subsequently be displaced downwards through the string to a level in the well at which a predetermined action is to be carried out by the tools. The displacement of the tools is obtained by connecting the tools to a pump plug, and creating a fluid pressure difference over the pump plug.
In general, a pump plug for use in this type of operations includes a cylindrical sleeve with a central axis, and an integral body of resilient material mounted on the sleeve, the body comprising a cylindrical part carrying a plurality of annular resilient sealing fins. The outer diameter of the sealing fins is selected such that when the plug is placed in a fluid conductor with circular cross-section, at least the rim portion of each fin seals against the inner wall of the fluid conductor. It will be appreciated that by closing off the flow passage through the sleeve and supplying fluid under pressure to one end of the pump plug for creating a pressure difference over the pump plug, the plug will be displaced through the conductor in a direction away from the supply of pressure fluid. A moderate pressure difference is already sufficient for displacing the pump plug. At such a pressure difference the outer rims of the resilient sealing fins stay in contact with the wall of the conductor thereby preventing the pressure fluid from by-passing the plug. The sealing fins, of at least the rim portions thereof are sufficiently flexible to follow variations in diameter of the flow conductor, such as are present at locations where sections of the flow conductor are coupled to each other, as well as in bends or curved portions of the conductor. Since the sealing fins will wear off when the plug is propelled through the conductor, the size of the sealing fins should be chosen such that a certain amount of wear is allowed before by-passing of the pressure or displacement fluid along the pump plug will occur.
When the pressure difference over the plug exceeds a predetermined value, the fins will loose their sealing position and the pressure fluid will by-pass the plug. By-passing of the pressure fluid may be necessary for a number of different purposes such as for propelling a plurality of pump plugs attached to each other, and for avoiding complete obstruction of flow conductors by pump plugs.
Apart from using the pump plug in TFL-operations, the plug may also be used in swabbing operations. In these latter operations a cable is connected to one end of the sleeve (that has the flow passage therethrough closed off against fluid flow), and the plug is (after weightening) lowered in the production tubing of a well. By pulling the cable upwards, the well pressure is lowered and the well will start to flow. This is a known technique which does not require any further detailed description thereof.
The known pump plugs designed for use in the above-mentioned operations are provided with sealing fins of cup-shaped configuration. In TFL-operations, two plugs are interconnected such that the cups are positioned in opposite directions. Hereby, the plugs will seal off against the inner surface of the flow conductor in opposite directions of plug motion. Unfortunately, it has been found in field operations, that the cups when positioned with the open ends thereof in the direction of travel, will easily be damaged when passing through portions of the flow conductor that have a reduced diameter. In passing through portions of the flow conductor where an abrupt decrease in diameter of the flow passage therethrough takes place, the damage of the cups may reveal itself in the form of cracks at the locations where the cups are connected to the cylindrical part of the integral body of resilient material. In extreme cases, the cups will even be torn from the integral body.
It will be appreciated that such damage will render the pump plug useless in TFL-operations since, once the plug has been pumped down to the bottom of a well, it cannot be returned to the surface by reversing the flow direction of the displacement fluid in the well, as the cups that are designed for returning the plug to the surface have been damaged during the downward travel, are consequently no longer able to seal efficiently against the inner wall of the flow conductor. The displacement fluid then by-passes the damaged cups and the pump plug stays down in the well.